UNNEST

The UNNEST function takes an ARRAY and returns a table with a row for each element in the ARRAY.

UNNEST(ARRAY) [WITH OFFSET]

SELECT
  *
FROM
  UNNEST([1, 2, 2, 5, NULL]) AS unnest_column

The optional WITH OFFSET clause provides an additional column containing the position of each element in the array (starting at zero) for each row produced by UNNEST.

SELECT
  *
FROM
  UNNEST([1, 2, 2, 5, NULL]) AS unnest_column WITH OFFSET AS `offset`

• UNNEST requires an ARRAY as an input but can return a table of any structure.

• Empty arrays and NULL as an input returns an empty table. An array containing NULL values will produce a row of NULL values.

You can use UNNEST to quickly create simple tables from arrays. For example:

SELECT
  *
FROM
  UNNEST(
       ARRAY<STRUCT<fruit STRING , number INT64>>[
         ('apples', 4), ('pears', 6), ('bananas', 2)
       ]
  )
AS simple_table;

You can do this with a CROSS JOIN. A cross join will take every individual element of your unnested array and join it back to its parent row. This will create multiple rows for each element of your array but you can then filter it down.

WITH fruit_baskets as 
(
  SELECT
    * 
  FROM
    UNNEST(
      ARRAY<STRUCT<fruit ARRAY<STRING>,basket STRING>>[
        (['bananas', 'apples', 'oranges'], 'basket 1'),
        (['bananas', 'oranges'], 'basket 2'),
        (['bananas', 'apples'], 'basket 3')
      ]
    )
  AS fruit
)

SELECT * 
FROM fruit_baskets 
CROSS JOIN UNNEST(fruit) as fruit_unnest